1. Field of the Invention
The present invention relates to a device for measuring a physical value and a vibrator.
2. Related Art Statement
A piezoelectric vibratory gyroscope utilizes a Coriolis force, which is generated when an angular movement is applied to a vibrating object in a direction perpendicular to the vibratory direction. Its mechanism is understood by using a dynamic model (For example, see “Handbook of Elastic Wave Device Technologies” (Danseiha-Sosi Gijutsu Handbook) published by Ohm, Inc., pp. 491 to 497).
The assignee has studied various applications of a vibratory gyroscope and has studied to use a vibratory gyroscope as a turning angular rate sensor employed in a vehicle control system of an automobile body based on a vehicle turning rate feedback system. Such a system detects the direction of a steering wheel using a turning angle of the steering wheel. Simultaneously, the turning rate of the car vehicle is detected by the vibratory gyroscope. The system finds a difference by comparing the direction of the steering wheel with the actual body turning velocity, and attains a stable vehicle control by correcting wheel torque and steering angle on the basis of this difference.
It is indispensable to detect the turning angular rate at a high precision in such control system. When the detection of turning angular rate at a high precision is tried, however, unnecessary dislocation, or movement, may be easily induced in the bending vibration arm of a vibrator. The unnecessary dislocation may directly contribute to an error in a detection signal from the arm. That is, in a vibratory gyroscope, driving vibration is excited in the driving vibration arm of a vibrator. When the vibrator is then turned, a detection electrode provided on a detection vibration arm detects a detection vibration induced in the vibrator. The thus obtained alternating current output signal is supplied into a detection circuit, and subjected to a process for reducing the effects of the driving vibration as much as possible to finally obtain an output signal corresponding to a turning angular rate. The output signal is normally output as a value of direct current. The unnecessary vibration and dislocation directly affect the measured value of turning angular rate to result in a noise, which is difficult to precisely remove.
The assignee has disclosed the idea of forming an elongate hole in a driving vibration or detection vibration arm of a vibrator, as described in Japanese patent publication 11-125528A. The applicant has tried to provide a through hole in a longitudinal direction of an elongate arm so as to lower the resonant frequency of the arm. It has further tried to provide driving and detection vibration electrodes on the inner wall surfaces facing the through holes to reduce the above described unnecessary dislocation and vibration.
However, the assignee has further studied and found that the magnitude of noise may be increased when a through hole is formed in the driving or detection vibration arm. For example, when a vibratory gyroscope is mounted in an automobile for controlling the direction of a car body of the automobile, the vibratory gyroscope is contained in a housing. The housing needs to be fixed onto the chassis of the car body of the automobile. In this case, an outer vibration (a linear acceleration) is transmitted to the vibrator from the chassis of the car body of the automobile. Such irregular linear acceleration or outer disturbance applied onto the vibratory gyroscope may result in an increase of noise level.